It is known that all offshore installations/units, an independent emergency source of electrical power is to be provided, which should be able to take over the supply of the emergency consumers, e.g. lighting and important communications and signalling systems, etc., when the main power supply fails; and the supply time period should meet the requirement from e.g. several hours to several days for different loads.
Offshore wind farm is one typical offshore application which is expected to grow in a steady pace and the installed power is expected to increase. The electrical system design should also comply with the requirements for emergency auxiliary load supply, including navigation lights, sensor and communication apparatus, ventilation and heating equipment etc., when the whole wind farm is disconnected from the onshore grid, e.g. in the circumstance of export cable failure.
Patent WO 2014082757 A1 discusses a method involving connecting a converter unit to an electrical network on network side. The electrical energy generated by a generator is fed into the electrical network via converter unit. The electrical energy is drawn from an energy storage unit by an auxiliary energy unit for an adjustable period of time if converter unit and auxiliary energy unit are disconnected from electrical network. The electrical energy generated by generator is then drawn by the auxiliary energy unit via the converter unit.
However, the reference suffers from the disadvantage: a separate machine is needed for charging the energy storage unit when its energy is consumed, and this will incur additional cost.